Ribbed lock washer affect on bolt torque values

[dtwo]

We are currently using M8 A2-70 Stainless steel bolts on one of our pieces of equipment. There have been several bolt failures throughout the life of the equipment. It's a high impact and force application. My original guess to the failure is bolt fatigue. One thing I want to make sure is the bolts are "tight" initially achieving the right amount of stretch and remain tight in order for the joint to absorb some of the shock.
Finally my question, I am using a torque value I found of 21.4 Nm along with a ribbed lock washer to prevent loosening. What affect does the conical ribbed washer have on the 21.4 Nm torque.

 

[Screwman]

It is hard to say exactly what the ribbed washer is doing without having actual clamp load testing of the assembly, but experience tells me tht it is eating up a significant amount of your torque, resulting in the low clamp load situation that you suspect.
One down and dirty way to see how close your seating torque is to where you want is take some assemblies to failure (using all production parts) and then take the average of the failure torques and compare 75% of that number to your seating torque. They should be pretty similar.

 

[BillPSU]

Is this a nut and bolt application or a threaded hole with a bolt?

Lock washers have been shown on many occasions to be ineffective in maintaining torque.

I was involved in testing a planetary axle design where the outboard planetary spindle was bolted to the axle. We were experiencing bolt failures with this connection. The solution was counterboring the top of the threaded holes in the axle housing allowing the bolts to stretch and not see shear loading.

Maintaining proper torque on a bolt is a different problem. In some equipment periodic inspection of bolt torque is required. Do you verify bolt torque on your preventative maintence process? Do you know if the bolts are loosening?

 

[dtwo]

Thanks for responding. In response to your questions:

1. socket head cap screws are threaded into a blind hole.
2. because we have cronic bolt failures in this area checking for loose bolts happens on a bi-weekly schedule.

 

[dtwo]

Anyone have an idea how much torque it takes to break M8 A2-70 Stainless Steel socket head cap screws? I am looking for tested values instead of theory.

 

[CoryPad]

It depends on how the torque is applied. Are you applying torsion to the screw only? Or, is the screw engaged in a nut? Is the screw tightened? What are the friction conditions (coatings, prevailing torque features)? Values could vary widely. If the screw goes through a clearance hole in a plate into a threaded hole/nut, if the screw has a lubricant, if there are no prevailing torque features, then it would take about 35 N[·]m. Since you seem to have a prevailing torque feature (and maybe no lubricant), then it may take closer to 50 N[·]m.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dtwo]

Cory,

To answer some of your questions:

non-lubricated
torsion only
bolt through a clearance hole into a blind tapped hole

Thanks for the response